This invention relates to dental and medical prosthetic devices, more particularly a dowel post for anchoring such prosthetic devices in position.
The invention will be particularly described with reference to dowel posts for anchoring dental crowns in dental restoration work, but it will be understood that the principles of the invention can be applied in other branches of dental or medical surgery or treatment wherein an anchor is required for securing a prosthetic device in position, for example, the securing in position of an artificial joint by means of a post anchored in a bone.
In dental restoration work, post crowns are generally mounted on a dowel post which is inserted and secured in a previously prepared hole in the root of the tooth to be crowned. Various designs and configurations have been proposed for the post itself and various methods have been used to secure the dowel post in position in the tooth root. One of the simplest techniques is to use a plain parallel-sided dowel post which is inserted into a previously prepared, parallel-sided blind hole in the root and secured therein by means of a cement. Tapered posts have also been proposed, as well as posts possessing both tapered and parallel sections, although, in general, such posts show less final retention strength then parallel-sided posts. Threaded posts have also been proposed, e.g. U.K. Pat. No. 1,092,982, but these have the disadvantage of requiring a thread to be tapped in the root of the tooth, a process which is difficult to carry out owing to the poor receptivity of dentine to the cutting of the thread therein. Threaded crowns also tend to work loose unless additionally secured by a cement, which brings further problems.
Generally speaking, where a cement is used two major problems arise, namely: obtaining an adequate distribution and thickness of cement between the post and the sides of the prepared hole in the tooth root, and secondly, the avoidance of hydrostatic pressure which may build up ahead of the post as it is inserted, either by virtue of an air bubble trapped ahead of the post, or by an accumulation of cement in the bottom of the hole due to the wiping action of the post on the walls of the hole as the post is inserted. Generally, the cement is applied either by lining the hole with cement before insertion of the post, or by applying the cement to the post before insertion or a combination of the two. The former technique, whilst usually obtaining a complete or substantially complete distribution of the cement around the post is particularly prone to the build-up of hydrostatic pressure ahead of the post as it is pushed into the prepared hole, and this at best will limit the depth to which the post can be inserted, but may also result in the fracture of the tooth root. Application of the cement to the post prior to insertion substantially avoids the problem of hydrostatic pressure due to the trapping of cement ahead of the post during insertion, but almost invariably the act of inserting the post leads to the wiping off of a substantial proportion of the cement from the post, with the result that an incomplete and uneven cement distribution is obtained, which in turn, results in low retention strength. Lining the hole is therefore the preferred technique, but at the risk of hydrostatic pressure build up in the tooth root. To relieve this pressure build-up, and also air pressure which may likewise build up due to the tight fit between the post and the hole, it has been proposed (Swiss Pat. No. 424,086) to provide a channel or groove in the surface of the post and extending longitudinally thereof to provide a passageway for venting excess cement and/or air pressure from in front of the post as it is inserted into the blind hole. This, however, still does not solve the problem of obtaining uniform cement distribution and preventing wiping-off of the cement during insertion.